1. Field of the Invention
The present invention relates to position indicating circuits generally and particularly to position indicating circuits for indicating the position of a nuclear reactor control rod.
2. Description of the Prior Art
Position indicating systems are generally known for indicating the position of a control rod in a nuclear reactor. Such known systems employ a permanent magnet mounted to the control rod drive mechanism to be movable internally of the reactor along with the control rod. A reed switch circuit is usually located externally of the nuclear reactor along a path corresponding with the operating length of the control rod and hence along the operating length of travel of the permanent magnet. As the control rod moves, different reed switches of the circuit are progressively actuated by the magnetic flux of the permanent magnet and yield appropriate output signals whose magnitudes are indicative of the position of the control rod. These output signals are used to limit the nuclear reactor power level under certain conditions.
Should any of the reed switches fail to be actuated, a faulty output signal results from the circuit and the operation of the nuclear reactor may be impaired. Should a reed switch fail to close in response to the magnetic flux of the magnet a zero output signal results from the circuit which signal is usually indicative of a fully inserted control rod position. Such a control rod position would falsely signal a group asymmetry condition that could require possible reactor power runback which is time consuming and expensive. On the other hand should a reed switch fail to open when the magnetic flux is removed the circuit would now provide two output signals, the signal from the failed open reed switch and the signal from the newly actuated reed switch. These two signals would combine to provide an output signal indicative of a completely different control rod position in the reactor than actual. This may also result in a false group asymmetry condition and cause a power runback. In such prior art systems once the fault was discovered the entire circuit had to be replaced entailing costly shut down and replacement.